To say Nikon has been conservative with mirrorless cameras would be an understatement. While most of its rivals have embraced mirrorless (even a cautious company like Canon takes them seriously), Nikon's efforts have been timid at best -- the 1 series is closer to point-and-shoots in spirit than a DSLR replacement. Thankfully, it's mending its ways. In an interview with Xitek, company R&D manager Tetsuro Goto says that any new Nikon mirrorless camera "must" be full-frame. Given that the firm's president has already said that a mirrorless camera is in the works, it's clear that this upcoming model will come with a giant sensor.
As for the reason why? Goto claims that competitors like Fujifilm, Olympus and Sony don't really cater to pro photographers with their mirrorless cams, and Nikon could fulfill that role. "There is no professional using their products," he says. We'd disagree strongly with that statement (the Sony A9 is very clearly aimed at pros), but it's true that most mirrorless cameras are aimed at enthusiasts and others who don't depend on photography as their main source of income.
Goto unsurprisingly isn't revealing specifics. However, Nikon has patented two full-frame lenses (a close-up 36mm f/1.2 and an ultra-bright 52mm f/0.9), so that will at least give you an idea of what to expect -- Nikon wants high-quality glass for portraits and other shots that pros like to take. This definitely won't be cheap, but it may scratch the itch for serious photographers who don't want to lug around a giant DSLR just to take print-worthy shots.